The invention relates to a device for producing a flowable reaction mixture of at least two flowable reaction components which react completely with each other to form solid substances or foam materials. Devices for producing flowable reaction mixtures are known in the art and generally comprise storage vessels, from which feed pipes lead via dosing pumps to a mixing head which contains a mixing chamber with ejector piston guided therein, into which mixing chamber feed pipes lead by means of injection nozzles, and a throttle slide arranged transversely to the mixing chamber in a guide bore, and also a discharge channel with cleaning piston. One such device is described in published European patent application 70,486.
A mixhead for preparing such flowable mixtures is also described in German Patent 2,327,269 (U.S. Pat. No. 3,975,128) and U.S. Pat. No. 3,975,128. In the mixhead described therein, immediately following the mixing chamber there is positioned at right angles a discharge channel which is intended to calm the reaction mixture and which can be cleaned by the corresponding ejector piston. In practice, attempts have also been made to use this ejector piston for the purpose of throttling the flow of reaction mixture issuing from the mixing chamber. In the case of reaction components which are not readily miscible, the throttling action needs to be of such strength that it is not possible to calm the flow of mixture sufficiently before it enters the mold cavity. This results in the reaction mixture spraying out of the discharge aperture of the discharge channel and not forming a closed flow front.
Another well-known mixing head (German Patent 2,612,812 (U.S. Pat. No. 4,141,470) and U.S. Pat. No. 4,141,470) has a transverse slide between the mixing chamber and discharge pipe by way of a throttling device, said transverse slide being provided with a through-flow aperture. By displacing this throttling device, it is possible to adjust the desired through-flow cross-section. For the purpose of cleaning the mixing chamber and the discharge pipe, the throttle slide is placed in the cleaning position so that the ejector piston can pass through the flow aperture. This mixing head has a relatively large overall length. A problem in this connection is the precise control of throttling device and ejector piston so as to ensure that these two elements do not collide. Also unfavorable is the large resulting overall length of the ejector piston, since in this connection there is a danger that a film of mixture remaining between ejector piston and the wall of the mixing chamber and discharge pipe may cause the ejector piston to adhere to this wall, which could cause the piston to become blocked. In order to prevent this, an inordinately large propulsive power has to be provided for the ejector piston. Because of the danger of the ejector piston breaking, it must have a certain minimum diameter for its length. For this reason the cross-sections of mixing chamber and discharge pipe cannot be arbitrarily small. A further disadvantage is that the cross-section of the mixing chamber and the cross-section of the discharge pipe must be correspondent to each each other.
Although the device according to published European patent application 70,486 does not exhibit the processing disadvantages of the aforementioned mixheads, it is however very costly to construct and is of unwieldy size.
In order to improve still further the rapid abatement of the mixture flow after throttling and to ensure that a device which is less costly to build and which is of small dimensions, is not susceptible to faults, is safe to operate and is capable of cleaning itself, even in the case of processing reaction components which are not readily miscible, it has been proposed according to published European patent application 498,227 (U.S. Pat. No. 5,143,946) to arrange the throttle slide in the guide bore concentrically with regard to the cleaning piston, whereby the guide bore protrudes in the direction of flow of the mixture beyond the discharge of the mixing chamber and in the mixing phase forms a ring channel with the front end of the cleaning piston, said ring channel passing into the discharge channel. In this way it has been possible to arrive at a device in which the mixture flow issuing from the throttle gap formed between the mixing-chamber discharge and the throttle slide cannot enter the discharge channel directly but first reaches a ring channel formed by the wall of the guide bore of the throttle slide externally and the cleaning piston which projects beyond the throttle slide. From this ring channel the reaction mixture then flows between the junction edge formed by the guide bore and discharge channel with the front edge of the cleaning piston as a downward flow, whereby the discharge channel forms the sink. As a result of the concentric arrangement of throttle slide and cleaning piston, this device is of very compact design; collisions of the pistons and the throttle slide are not possible. Surprisingly, no material accumulates at the ring-shaped shoulder at the interface between guide bore and discharge channel, since mixture residues are completely pushed away by the thrusting of the throttle slide. It has been shown with this device, however, that if the throttle slide and the ejector piston are unfavorably positioned, the desired laminar sinking flow does not set in or only sets in approximately, since the throttled jet can spurt directly out of the mixing chamber into the discharge channel.
It is the object of the present invention to improve the device described in published European patent application 498,227 (U.S. Pat. No. 5,143,946) in such a way that the desired laminar sinking flow develops also in the case of extreme throttling.